Preheat oven to 350°F. Coat a 9-inch round cake pan with cooking spray. Line the pan with a circle of wax paper.
Whisk flour, granulated sugar, cocoa, baking powder, baking soda and salt in a large bowl. Add buttermilk, brown sugar, egg, oil and vanilla. Beat with an electric mixer on medium speed for 2 minutes. Add hot coffee and beat to blend. (The batter will be quite thin.) Pour the batter into the prepared pan.
Bake the cake until a skewer inserted in the center comes out clean, 30 to 35 minutes. Cool in the pan on a wire rack for 10 minutes; remove from the pan, peel off the wax paper and let cool completely. Dust the top with confectioners’ sugar before slicing.
Tips & Notes

Ingredient Note: Whole-wheat pastry flour, lower in protein than regular whole-wheat flour, has less gluten-forming potential, making it a better choice for tender baked goods. You can find it in the natural-foods section of large super markets and natural-foods stores. Store in the freezer.

Tip: No buttermilk? You can use buttermilk powder prepared according to package directions. Or make “sour milk”: mix 1 tablespoon lemon juice or vinegar to 1 cup milk.